The Bluetooth standard is a specification directed to Wireless Personal Area Networks (WPAN). Bluetooth provides short-range radio links to replace wires between computers and their peripherals, cell phones and ear pieces, etc. Typically, Bluetooth products are designed for transmitting relatively small amounts of data (at 1 Mbps) over short distances (up to 10 meters). Consequently, Bluetooth products do not require much power to operate. This conserves battery life. However, the low signal power is susceptible to noise and other factors that could interfere with and/or otherwise degrade the signal. In order to increase reliability, without consuming more power, Bluetooth utilizes a frequency hopping scheme. The signal intentionally changes or “hops” to different frequencies when transmitting or receiving data packets. This frequency hopping scheme is implemented by 79 hops displaced by 1 MHz, from 2.402 GHz to 2.480 GHz. Hopping in and out of a continuous range of frequencies affords the established communication link an opportunity to recover from errors and also makes the link more robust.
The Bluetooth protocol requires that a connection, also referred in Bluetooth technology as a pico-net, is first established between two or more units. The master-unit determines a frequency-hopping scheme. This scheme is then transmitted to the other units. The frequency selection scheme consists of two parts: selecting a sequence and subsequently mapping the sequence onto hop frequencies. Consequently, the frequency hopping scheme requires that the Bluetooth radio be able to operate at hopped frequencies. It also requires that when a hop to a new frequency occurs, the radio recognizes the change and settles quickly to it. The faster settling time is important because it translates into a shorter delay when hopping between frequencies. For Bluetooth and other wireless systems, the specified settling time can be in the range of 20 microseconds.
An ideal method for implementing a frequency synthesizer that meets the requirements of Bluetooth takes the form of a Phase Locked Loop (PLL). Besides inherently having a fast settling time, a PLL frequency synthesizer has a relatively high degree of stability and accuracy as compared to other forms of local oscillators. Furthermore, PLL frequency synthesizers are easily controlled by digital circuitry, such as microprocessors. It is for these reasons that a PLL frequency synthesizer is used in virtually every Bluetooth device.
In many instances, a capacitor array is implemented in order to provide for a variable PLL frequency synthesizer that can be used to synthesize frequencies over a specified band of frequencies. In order to lock a variable frequency synthesizer to a desired frequency, the appropriate capacitor is selected for use from the given array. There exist two methods for choosing the appropriate capacitor. In a binary search plus an optional linear search method (the “first” method), each time the channel is changed, the synthesizer performs a binary search on the capacitor array in order to find the correct setting. The second method uses a “Boundary Search” process. On power up, the synthesizer is calibrated by sweeping the entire frequency band and saving the frequencies whereby the capacitor setting needs to change by 1 LSB. This requires a large table of frequency versus capacitor settings.
The first method suffers from the fact that the binary search on each channel takes a long time to perform. In the second method, the required calibration time is prohibitive. Furthermore, the second method requires a large amount of silicon storage, which increases cost and size. Thus, although the two approaches offer solutions, they nonetheless suffer disadvantages, and neither one is optimal.